<span class="field" data-fullText="This is a span element">This is a</span>

$('.field').hover(function () {
    console.log('using prop(): ' + $(this).prop('data-fullText'));
    console.log('using data(): ' + $(this).data('fullText'));
});

$(this).attr('data-fullText')

data-fulltext="This is a span element"

$(this).data('fulltext')

$('.field').hover(function () {
    var value=$(this).attr('data-fullText');
  $(this).html(value);
});

$('.field').hover(function () {
    var value=$(this).data('fulltext');
  $(this).html(value);
});